1. Field of the Invention
The present invention relates to novel liquid crystalline compounds useful for electrooptical liquid crystal display materials, and more particularly, to novel liquid crystalline compounds exhibiting suitable various physical properties and liquid crystalline compositions comprised of the novel liquid crystalline compounds. In particular, The present invention relates also to novel liquid crystalline compounds possessing low viscosity (high responsibility) and adequate magnitude of values of optical anisotropy in addition to characteristics of steep threshold voltage and good compatibility with other liquid crystalline compounds and to novel liquid crystalline compositions composed of the novel liquid crystalline compounds as well as liquid crystal display elements.
2. Description of the Prior Art
From the past, a great number of compounds has been developed to have liquid crystalline characteristics. Such liquid crystalline compounds are required to possess various characteristics, and the following characteristics are above all regarded important:
1) characteristics of a steep threshold voltage,
2) shorter response time,
3) smaller electric power for driving,
4) a wider range of working temperature (good compatibility with other liquid crystalline compounds), and
5) chemical stability.
In the characteristics 4), such properties are involved that the melting point is low and that crystallization, precipitation of crystals or phase separation like precipitation of smectic phase is difficult to take place.
In order to exhibit such favorable characteristics, liquid crystalline compounds as constituent are required to possess the following characteristics:
a) higher ration of elastic constants (K33/K11),
b) lower viscosity (Phys. Lett., 39A, 69(1972),
c) In case of adding to a liquid crystalline composition, a temperature range for nematic phase is not shortened or phase separation such as precipitation of crystals, smectic phase or the like is hard to take place,
d) smaller ratio of elastic constants (K) (Mol. Cryst. Liq. Cryst., 12, 57 (1970), and
e) adequate magnitude of values of optical anisotropy and values of dielectric anisotropy (Appl. Phys. Lett., 38 (7), 497).
Among the aforesaid characteristics required, what is required for enhancing display quality of STN liquid crystalline compositions and STN liquid crystal display elements most widely utilized at the present time is steepness in characteristics of threshold voltage. It is known that this steepness is greatly caused by the ratio of elastic constants (K33/K11) (Proc. of the Japan Display, 388 (1986). It is also known that a compound having a larger ratio of elastic constants shows characteristics of steep threshold voltage. Thus, the use of a liquid crystalline composition possessing better characteristics of threshold voltage can provide a liquid crystalline composition and a liquid crystal display element of better display quality. In STN liquid crystalline materials, this steepness is one of the most important characteristics in practical use.
It is well known that compounds having an alkenyl group in a side chain thereof has a greater ratio elastic constants (K33/K11), and various compounds are proposed hitherto [Mo. Cryst. Liq. Cryst., 122, 241 (1985), ibid. 131, 10 (1985), ibid. 149, 359 (1987), and ibid. 165, 405 (1988)]. However, the ratio of elastic constants of these compounds are not as yet large enough for practical use. Under the above circumstances, therefore, there is a great demand in this art for developing a new liquid crystalline compound which has a satisfactorily large ratio of elastic constants as well as a liquid crystalline composition containing such compounds. Till now, such demand has not yet been fulfilled.